In recent years, portable information terminals such as cellular phones, laptop computers, and PDAs have become increasingly smaller and more light-weight, and higher capacity has been required for batteries serving as driving power sources for portable information terminals. Lithium-ion batteries, which are charged and discharged through movement of lithium ions between the positive and negative electrodes, have high energy density and high capacity and thus are widely used as the driving power sources for the above portable information terminals.
As portable information terminals have been made to have greater functionality such as video-replaying functions and game functions, there has been a trend toward a further increase in the power consumption of portable information terminals. Thus, a further increase in the capacity of batteries has been strongly demanded. Examples of methods for increasing the capacity of nonaqueous electrolyte secondary batteries include a method for increasing the capacity of the active material, a method for increasing the amount of active material packed per unit volume, and a method for increasing the charge voltage of a battery. However, when the charge voltage of a battery is increased, the electrolyte tends to decompose. In particular, when such a battery is stored at high temperature or is repeatedly subjected to a charging-discharging cycle at high temperature, problems such as a decrease in the discharge capacity are caused.
In view of the foregoing, as described below, it has been proposed that a mixture of lithium cobalt oxide and lithium nickel cobalt manganese oxide be used as a positive electrode active material.
(1) There has been proposed a nonaqueous secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains at least LixCoO2 and LiyNisCotMuO2 (M is B, Mg, or the like) as positive electrode active materials, and the content of LiyNisCotMuO2 is 10% to 45% by mass relative to the total amount of LixCoO2 and LiyNisCotMuO2 (refer to PTL 1 below).
(2) There has been proposed a mixture that contains a lithium-cobalt composite oxide obtained by dissolving a first minor component selected from Al and Fe and a second minor component selected from Zr and Ti and a lithium-nickel-cobalt-manganese oxide at a ratio of 95:5 to 70:30 (refer to PTL 2 below).
(3) There has been proposed coating of a surface of a positive electrode active material with a compound (refer to PTL 3 below).